1. Field of the Invention
The present invention relates to a semiconductor crystal, and a method and apparatus of producing the same. Particularly, the present invention relates to a semiconductor crystal for use in production of a GaAs substrate and the like employed in an optical device, an electronic device, and the like, and a method and apparatus of producing such a semiconductor crystal.
2. Description of the Background Art
A GaAs crystal, for example, among the semiconductor crystals, is produced industrially by the pulling method (LEC method), the horizontal boat method HB method, HGF method), and the vertical boat method (VB method, VGF method). The pulling method and the vertical boat method are particularly advantageous over the horizontal boat method for producing a single crystal since the yield is improved due to the cross section of the obtained crystal being circular identical to that of the substrate and since the diameter can easily be increased due to the symmetry of the thermal environment in the growth system.
As an example of an apparatus of producing a semiconductor crystal, an apparatus having a carbon heater and a crucible that stores material melt placed in a stainless steel-made high pressure chamber is known. Such an apparatus is used in the LEC, the VB, or the VGF method.
FIG. 5 shows an example of such an apparatus using a stainless steel-made high pressure chamber. A cross sectional view of a schematic structure of an apparatus of producing a semiconductor crystal employed in the pulling method is shown.
Referring to FIG. 5, the apparatus includes a crucible 2 supported by a lower shaft 4, and a carbon heater 3 in a stainless steel high pressure chamber 11. A heat insulator 5 is provided between carbon heater 3 and stainless steel high pressure chamber 11 to prevent damage of chamber 11 caused by the heat of carbon heater 3.
In growing a crystal using such an apparatus, crucible 2 is first filled with GaAs material to prepare material melt 60 by the heat from carbon heater 3. The surface of material melt 60 is encapsulated by a liquid encapsulation material 70 in order to prevent evaporation of As from material melt 60. A pull shaft 14 having a seed crystal 55 attached at the leading end is pulled upward as indicated by the arrow to effect crystal growth under a high pressure atmosphere. Thus, a GaAs single crystal 50 is obtained.
FIG. 6 shows another example of an apparatus employing a stainless steel high pressure chamber. A cross sectional view of a schematic structure of an apparatus of producing a semiconductor crystal employed in the vertical boat method such as the VB or VGF method is shown.
Referring to FIG. 6, the apparatus has seed crystal 55 placed at the lower portion of crucible 2. By moving lower shaft 4 downwards as indicated by the arrow or by shifting the temperature distribution, material melt 60 is solidified from seed crystal 55 sequentially upwards for crystal growth. The remaining structure is similar to that of the apparatus of FIG. 5. Therefore, description thereof will not be repeated.
As another example of an apparatus for crystal growth, an apparatus is known having a crucible or boat that stores material melt sealed in vacuum in a quartz ampoule, which is heated fiom the outer side. Such an apparatus is used in the horizontal boat method including the HB or HGF method, and in the vertical boat method including the VB or VGF method.
FIG. 7 is a sectional view showing a schematic structure of such an apparatus employing a quartz ampoule.
Referring to FIG. 7, the apparatus has crucible 2 sealed within a quartz ampoule 21. A heater 3 such as of kanthal is provided outside ampoule 21. Quartz ampoule 21 is supported by lower shaft 4. A crystal is grown by moving lower shaft 4 downwards as indicated by the arrow or by shifting the temperature distribution.
As a method of synthesizing the GaAs material for crystal growth, the As injection method of effecting reaction between the Ga in the crucible and the arsenic vapor generated by controlling the temperature of the As source outside the crucible, and the method of charging both the Ga and the As into the crucible together and raising the temperature for a direct reaction are known. Both methods are carried out in a high pressure chamber under liquid encapsulation. It is particularly noted that the latter requires high pressure of several ten atmospheres since the arsenic vapor pressure becomes considerably high.
Production of a GaAs polycrystal is carried out by cooling the material obtained as described above. Production of a single crystal is carried out by the method of charging the prepared polycrystal into a crucible as the material, and the method of growing a single crystal subsequent to the raw material synthesization.
There is the demand for a larger semiconductor crystal as the integration density of a semiconductor device becomes higher. Currently, a GaAs crystal with 4 inches in diameter is of practical usage as for a compound semiconductor crystal. The need of increasing the size of such a compound semiconductor crystal has become greater to induce various research and development. However, there are many problems on the mass production of a large compound semiconductor crystal. Production of a large compound semiconductor crystal greater than 4 inches is not yet practical.
For example, when the stainless steel high pressure chamber shown in FIG. 5 or 6 is used, a heat insulating layer must be inserted between the heater and the stainless steel chamber. Accordingly, the size of the furnace becomes larger, thus increasing the furnace cost.
According to the method shown in FIG. 5 or 6, carbon (or graphite) is employed for the heater material. Heating up to a temperature as high as approximately 1300.degree. C. is required in preparing the material melt since the melting point of GaAs is 1238.degree. C. Here, the vapor pressure of carbon is small even at the high temperature of approximately 1300.degree. C. Therefore, carbon is suitable to be used for the heater. However, carbon is an element that is electrically active in a GaAs semiconductor single crystal. Therefore, the concentration of the carbon must be controlled in order to obtain a single crystal of high quality. When the method employing the stainless steel high pressure chamber shown in FIG. 5 or 6 is to be carried out, various measures must be taken to control the electrical properties of GaAs crystal since the carbon and the synthesized GaAs reside in the same space. Thus, there is a problem that the furnace cost is increased.
In the case where the quartz ampoule shown in FIG. 7 is used, there is a problem in that it is difficult to produce a large crystal by charging a great amount of the material since so much heat must be applied that there is a possibility of deformation or breakage of the ampoule. There is also a problem that the material cannot be synthesized, in situ, since the ampoule is sealed, baring the application of the As injection method. There is also the problem that the atmospheric gas cannot be controlled after the ampoule is sealed.
Japanese Patent Laying-Open No. 7-221038 discloses an example of using silicon carbide in the annealing furnace of a semiconductor. However, this publication is silent about the advantage of using such an apparatus in the growth of a single crystal.
Japanese Patent Laying-Open No. 2-233578 discloses an apparatus of growing a semiconductor single crystal such as GaAs according to the pulling method with the entire apparatus placed in a stainless steel chamber. This apparatus is characterized in that a solid gasket is used as the heat-proof sealing material since the chamber made of silicon carbide is subjected to high temperature. However, the heat-proof sealing member has poor airtightness, so that sufficient difference between the inside pressure and the outside pressure cannot be achieved.
Japanese Patent Laying-Open No. 2-120292 discloses an embodiment that employs silicon carbide for the crucible. However, this publication is completely silent about using silicon carbide for the reactor tube.
Thus, the need of a large GaAs semiconductor single crystal with at least 6 inches in diameter has become greater in response to the semiconductor device scaled to higher integration density. A semiconductor crystal of a high quality is required at low cost.